The purpose of this project is to provide new knowledge of the auditory mechanisms of the inner ear. We have found that mechanically dissociated and isolated outer hair cells of the organ of Corti of the guinea pig when electrically stimulated show shape changes that are not dependent on metabolically based energy. These changes could be visualized only through the use of video enhanced microscopy. There is evidence that an electro-osmotic mechanism was underlying these changes. We have continued immunocytochemical studies, using small mammals. We have used several different antibodies, among them a highly specific antibody against GABA. We have studied the distribution of immunoreactivity through light microscopy, including video enhanced microscopy, and electron microscopy. We have found that axons and endings of efferent neurons in the organ of Corti contain GABA-like immunoreactivity with a distribution similar to that of GAD-evidence that certain efferent neurons projecting to the organ of Corti, some of them to outer hair cells, are GABA-ergic. The evidence indicates, also, that such a GABA-ergic population of neurons is not part of the lateral and medial efferent systems. We have determined that dynorphin and an enkephalin are co-contained in lateral olivocochlear neurons.